Luminaries with automated and remotely controllable functionality are well known in the entertainment and architectural lighting markets. Such products are commonly used in theatres, television studios, concerts, theme parks, night clubs and other venues. A typical product will typically provide control over the pan and tilt functions of the luminaire allowing the operator to control the direction the luminaire is pointing and thus the position of the light beam on the stage or in the studio. Typically this position control is done via control of the luminaire's position in two orthogonal rotational axes usually referred to as pan and tilt. Many products provide control over other parameters such as the intensity, color, focus, beam size, beam shape and beam pattern. The products manufactured by Robe Show Lighting such as the ColorSpot 1200E are typical of the art.
Considering as an example, the use of such a product in a theatre, it is common for an automated luminaire to be situated at some considerable distance from the stage, perhaps 50 feet or more. At such a distance very small positional movements of the luminaire will produce a correspondingly large movement of the light beam where it impinges on the stage. In the example given of a 50 foot throw a displacement of 1 inch on the stage would be caused by a change in angle of either of the pan and tilt axes of the light of only 0.1 degree. If we consider that a positional accuracy of the light on the stage of less than 1 inch is desirable we can see that a very high degree of rotational accuracy is desirable for the pan and tilt systems.
FIG. 1 illustrates a typical multiparameter automated luminaire system 10. These systems typically include a plurality of multiparameter automated luminaries 12 which typically each contain on-board a light source (not shown), light modulation devices, electric motors coupled to mechanical drives systems and control electronics (not shown). In addition to being connected to mains power either directly or through a power distribution system (not shown), each luminaire is connected is series or in parallel to data link 14 to one or more control desks 15. The luminaire system 10 is typically controlled by an operator through the control desk 15.
FIG. 2 illustrates different levels of control 20 of a parameter of the light emitted from a luminaire. In this example the levels are illustrated for one parameter: pan (typically movement in a horizontal plane). The first level of control 22 is the user who decides what he wants and inputs information into the control desk through typical computer human user interface(s). The control desk hardware and software then processes the information 26 and sends a control signal to the luminaire via the data link 14. The control signal is received and recognized by the luminaire's on-board electronics 28. The onboard electronics typically includes a motor driver 30 for the pan motor (not shown). The motor driver 30 converts a control signal into electrical signals which drive the movement of the pan motor (not shown). The pan motor is part of the pan mechanical drive 32. When the motor moves it drives the mechanical drive 32 to drive the mechanical components which cause the light beam emanating from the luminaire to pan across the stage.
In some systems it may be possible that the motor driver 30 is in the control desk rather than in the luminaire 12 and the electrical signals which drive the motor are transmitted via an electrical link directly to the luminaire. It is also possible that the motor driver is integrated into the main processing within the luminaire 12. While many communications linkages are possible, most typically, lighting control desks communicate with the luminaire through a serial data link; most commonly using an industry standard RS485 based serial protocol called commonly referred to as DMX-512. Using this protocol of the control desk typically transmitting a 16 bit value for pan and a 16 bit value for tilt parameters to the luminaire. Sixteen (16) bits provides for 65,536 values or steps which provides plenty of controller instruction accuracy for a typical application. If the total motion around and axis is 360 degrees then a 16 bit instructions can provide accuracy of instruction of approximately 0.005 degrees (360°/65,536). With this level of accuracy in the control instructional portion of the control system, the limiting factor in controlling the accuracy of the luminaire's motion predominantly lies with the mechanical systems used to move the pan and tilt axes.
One particular problem inhibiting and limiting the accuracy and repeatability of the movements of an automated luminaire is the mechanical hysteresis or backlash in the mechanical drive system. It is typical in such products to use a single motor or a pair of motors connected to the driven pan and tilt output shafts through either a belt drive or through a direct geared system. In both cases there is inevitably an amount of backlash or slippage or shifting which induces hysteresis in the system. This is particularly noticeable in use when a luminaire returns to a known and pre-recorded position from different directions. A clockwise pan to a stop position will allow the slack in belts or tolerance in gears and mountings to accumulate on one side of the system while an counter-clockwise pan to the same stop position will accumulate that same slack and tolerance on the other side thus giving an effective positional error of approximately two time (2×) the actual slack
Additionally there is a need to reduce the transfer of noise and vibration from the motor or motors to the chassis or mounting structure of the automated luminaire. This may be achieved through the use of resilient mounting pads or points as dampeners. Such resilient dampeners are effective at reducing vibration however they introduce a further source of hysteresis due to the inherent flex of the resilient materials.
Various systems have offered solutions to hysteresis. One solution is to provide deliberate dampening or friction to the system to smooth and minimize slack and tolerances. In practice such systems are difficult to control and difficult to manufacture repeatedly and consistently. Additionally any deliberate addition of friction will of necessity increase the power and size of motors needed.
Other solution utilizes highly accurate position sensors on the driven or output shaft of the device rather than, as is more common with servo systems, on the motor or driver shaft. Such systems are expensive to manufacture and may require significant processing power for each motor to ensure that smooth accurate movement occurs without hunting or overshoot.
Other system utilize ‘hunting’ or ‘backstepping’ techniques where the system homes in on the final desired position by taking small controlled steps towards it while monitoring the position accurately. Such a system is disclosed in U.S. Pat. No. 5,227,931 to Misumi which covers an anti-hysteresis system by backstepping. This system is slow to operate, requires an accurate sensor on the driven shaft and produces motion in the driven shaft while the final position is sought. It is important in theatrical applications that the driven shaft moves rapidly and accurately to its final position with no visible oscillation or hunting to find its resting point. Any such motion would be noticeable and distracting to the audience.
A yet further solution is to oscillate the output shaft about its final position to equalize any stress, slack or tolerance in the drive system and center the shaft. U.S. Pat. No. 5,764,018 to Liepe et al. uses a ‘shaking’ system where reducing oscillations center the driven shaft. This methodology has the disadvantage in that it gives significant and noticeable movement in the output not appropriate for the entertainment lighting application.
While the Misumi and Liepe systems may eventually and consistently get to the right position, the process of getting there may be worse than the hysteresis problem they solve in an automated luminaire application.
A still further solution is to use sensors for feedback with a pre-measured correction factor for hysteresis. U.S. Pat. No. 5,374,883 discloses such a system and uses a pre-determined correction for a particular sensor. Such a system is difficult to maintain as any changes in the system due to wear and tear or replacement of parts such as belts will require re-calibration and adjustment of the compensation system.
U.S. Pat. No. 6,580,244 to Tanaka et al discloses using two servo motors driven antagonistically to ensure tension is always in the same direction in the drive chain to avoid backlash. Although this provides good control of backlash when the system is always rotating in one direction to its final position, it doesn't cope as well with a system which has no prior knowledge of that direction and that can be required to travel to the same target position from either direction interchangeably. Accurate servos with sensors or encoders are still required for final positioning.
There is a need for a system which can provide hysteresis and backlash control to ensure accurate positioning of an automated luminaire motion control system without the necessity for accurate position sensors.